Hoists that lift loads in a vertical direction are used in many industries for a variety of applications. Single lift hoists are commonly used for heavy equipment and parts lifts for construction, architectural and industrial uses such as manufacturing plants, steel mills and transport loading facilities. These applications generally do not involve raising a load directly over people for safety reasons.
For theatrical settings, athletic and entertainment arenas, overhead lifting with higher safety standards are routinely required because hoists are lifting loads directly over human beings. For applications where loads are lifted above people, a plurality of lifts are generally required to meet applicable safety regulations.
Live performances in a theater typically employ a number of curtains and backdrops to convey to the audience different settings, environments, moods, and the like. These curtains and backdrops must be changed throughout the course of a performance within a fairly short time frame without interrupting the performance. Typically this is done by raising a particular backdrop above the stage and out of sight of the audience when it is not being used. When a particular backdrop is needed, it is lowered into place on the stage.
Theatrical backdrops and curtains are typically suspended from battens, which are pipes or trusses that span the width of the stage. Battens can be 20 feet or more in length, depending on the size of the stage. As should be apparent, the weight of the battens and the items suspended from them can have substantial weight. As the weight of the load increases so does the power required to raise the load. Counterweights are employed to balance the load of the batten and its associated load. Battens and their associated counterweights are manually lifted and lowered. In these types of systems, a rope is tied to a counterweight and the batten is manually raised or lowered, then tied off to a pin rail mounted to a wall adjacent the stage area. However, if the load is not closely balanced, excessive power may be required to move the load. Alternatively, the system may get out of control, dropping the load or the counter-weight, causing injury or death to people nearby and/or collateral damage.
Typical motorized hoists and winches have a grooved drum for winding and unwinding the cable attached to the battens. One or more grooves are typically disposed in a helical arrangement about the drum. A cable is fixed to the drum and disposed in the groove when it is wound about the drum. As the cable is unwound, the cable leaves the drum and passes over one or more sheaves to change the orientation of the cable from the drum to the batten. The angle at which the cable pays off the drum is the fleet angle, defined as the angle between the centerline of the groove on the drum and the cable coming off the drum. The fleet angle should be kept to a minimum because increasing the fleet angle results in increased wear on the cable and drum. Therefore it is desirable to minimize the fleet angle to prolong cable and drum life.